The pistons of rotary internal combustion engines of the trochoidal type, commonly known as Wankel engines, must have grooves cut in their planar end faces to receive the end face seals by means of which the pistons are sealed relative to the cavities in which they rotate. The shape of such grooves makes them very difficult to machine. Such grooves are uniformly curved from end to end as an arc of a circle, and they are very narrow and deep, commonly having a depth to width ratio in the neighborhood of four to one. Typically, such grooves are 0.04 inch wide by 0.16 inch deep.
Heretofore, the common practice has been to machine the grooves by means of a miniature end mill which may be no larger in diameter than the groove width. Tool life is short, machining time is uneconomically long, and the miniature end mill often produces an inadequate finish on the sides of the grooves which makes sealing difficult and tends to permit undesirable gas leakage.
An alternative method is described in U.S. Pat. No. 3,799,706, in which the rotor is made in two parts which are spaced to define the two sides of each groove. It is difficult to provide a good piston finish; and in addition the parts must be press fitted and it is expensive to do so because of the peculiar shape of the piston.